159
Chapter seven: Water balance
isotopes at multiple atomic positions. The advantage of this technique is that the positional
tracer enrichment can be used to determine the isotopic flux within multiple metabolic
pathways simultaneously during a single tracer study (i.e., Champagne et al. 2012).
7.3.3 Genome analysis
While inferences of functional relevance based on genetic analyses such as comparing
genomes among species requires a precautionary approach, the extensive information
obtained from genome-scale analyses should be appreciated. The molecular analyses that
allow for comparisons of gene sequences is indispensable and can provide insight to the
nature and extent of selective pressures that contributed to the evolution of certain osmoregulatory mechanisms. Such studies have been performed in cetaceans focusing on genes
related to proteins that contribute to osmoregulation. In 11 cetacean species, genes coding
for proteins involved in osmoregulation demonstrated positive selection with functional
changes in their amino acid sequences (Xu et al. 2013; Yim et al. 2014). Positive selection of
AQP2 and urea transporter-A (SLC14A2) in various cetaceans suggests that the evolution
of these genes was an important event in the development of enhanced capacity for water
and urea transport in the renal tubules (Xu et al. 2013) that can contribute to increased
urine concentrating ability. A series of positively selected amino acid residues identified
in angiotensinogen and ACE (two principal proteins of RAAS) of cetaceans was described
suggesting that the evolution of RAAS was a significant adaptation for the maintenance
of water and electrolyte balance in response to a hyperosmotic environment (Xu et al.
2013). Furthermore, functional changes in the sequences of five genes encoding for RAAS
proteins were reported in cetaceans (Yim et al. 2014). Nery et al. (2013) demonstrated that
genes related to proteins facilitating renal development (SMAD1, NPNT, LEF1, SERPINF1,
AQP2) are positively selected in bottlenose dolphins when compared to the cow lineage.
Collectively, these data suggest that the radiation of mammals to a marine environment
was associated with the evolution of genes encoding for proteins related to osmoregulation. While the comprehensive, panoramic investigations of genome-wide analyses provide
a wealth of information, inferences of physiological mechanisms based on such discoveries
need to be made with the appropriate precautions and confirmed by functional studies.
7.4 Unresolved questions
Plasma osmolality of marine mammals is maintained at relatively higher levels, attributed
to higher concentrations of plasma electrolytes and urea. The difference in average plasma
osmolality between some terrestrial and marine mammals can be as much as 75 mOsm/L,
and the average concentration of plasma Na + (155 mEq/L) for marine mammals would
be indicative of dehydration in humans. What osmoregulatory mechanisms have marine
mammals evolved to facilitate resistance to the detriments of such elevations in the osmotic
and electrolyte content of their extracellular fluid? While the effects of ion channels and
organic molecule transporters, organic anions and cations, and the kinetics of intra- and
extracellular water all likely contribute, research in these areas in marine mammals under
varying conditions are scarce or non-existent. As Ridgway and Venn-Watson (2010) have
suggested, the end-product of protein metabolism (i.e., urea and maybe ammonia) to regulate plasma osmolality via the concentration of urine is an relatively unexplored area of
interest in marine mammals, and further investigation along these lines has the potential to reveal very interesting and unique discoveries. The specific contribution of urea to
osmoregulation remains unclear in marine mammals. Regarding the hormonal control of
Chapter seven: Water balance
isotopes at multiple atomic positions. The advantage of this technique is that the positional
tracer enrichment can be used to determine the isotopic flux within multiple metabolic
pathways simultaneously during a single tracer study (i.e., Champagne et al. 2012).
7.3.3 Genome analysis
While inferences of functional relevance based on genetic analyses such as comparing
genomes among species requires a precautionary approach, the extensive information
obtained from genome-scale analyses should be appreciated. The molecular analyses that
allow for comparisons of gene sequences is indispensable and can provide insight to the
nature and extent of selective pressures that contributed to the evolution of certain osmoregulatory mechanisms. Such studies have been performed in cetaceans focusing on genes
related to proteins that contribute to osmoregulation. In 11 cetacean species, genes coding
for proteins involved in osmoregulation demonstrated positive selection with functional
changes in their amino acid sequences (Xu et al. 2013; Yim et al. 2014). Positive selection of
AQP2 and urea transporter-A (SLC14A2) in various cetaceans suggests that the evolution
of these genes was an important event in the development of enhanced capacity for water
and urea transport in the renal tubules (Xu et al. 2013) that can contribute to increased
urine concentrating ability. A series of positively selected amino acid residues identified
in angiotensinogen and ACE (two principal proteins of RAAS) of cetaceans was described
suggesting that the evolution of RAAS was a significant adaptation for the maintenance
of water and electrolyte balance in response to a hyperosmotic environment (Xu et al.
2013). Furthermore, functional changes in the sequences of five genes encoding for RAAS
proteins were reported in cetaceans (Yim et al. 2014). Nery et al. (2013) demonstrated that
genes related to proteins facilitating renal development (SMAD1, NPNT, LEF1, SERPINF1,
AQP2) are positively selected in bottlenose dolphins when compared to the cow lineage.
Collectively, these data suggest that the radiation of mammals to a marine environment
was associated with the evolution of genes encoding for proteins related to osmoregulation. While the comprehensive, panoramic investigations of genome-wide analyses provide
a wealth of information, inferences of physiological mechanisms based on such discoveries
need to be made with the appropriate precautions and confirmed by functional studies.
7.4 Unresolved questions
Plasma osmolality of marine mammals is maintained at relatively higher levels, attributed
to higher concentrations of plasma electrolytes and urea. The difference in average plasma
osmolality between some terrestrial and marine mammals can be as much as 75 mOsm/L,
and the average concentration of plasma Na + (155 mEq/L) for marine mammals would
be indicative of dehydration in humans. What osmoregulatory mechanisms have marine
mammals evolved to facilitate resistance to the detriments of such elevations in the osmotic
and electrolyte content of their extracellular fluid? While the effects of ion channels and
organic molecule transporters, organic anions and cations, and the kinetics of intra- and
extracellular water all likely contribute, research in these areas in marine mammals under
varying conditions are scarce or non-existent. As Ridgway and Venn-Watson (2010) have
suggested, the end-product of protein metabolism (i.e., urea and maybe ammonia) to regulate plasma osmolality via the concentration of urine is an relatively unexplored area of
interest in marine mammals, and further investigation along these lines has the potential to reveal very interesting and unique discoveries. The specific contribution of urea to
osmoregulation remains unclear in marine mammals. Regarding the hormonal control of